Tag Archives: mass

[Apologies: due to a computer glitch, the figure in the original version of this post was not the most up-to-date, and had typos, now fixed.]

On Tuesday, the New York Times Editorial page ran an Op-Ed about dark matter… and although it could have been worse, it could certainly have been better. I do wonder why these folks don’t just call up an expert and confirm that they’ve actually got it right, before they mislead the public and give scientists a combination of a few giggles and a headache.

Here is the last paragraph from the Times:

“This experiment is probing a major hole in the way we understand the cosmos. Roughly speaking, the force of gravity in the universe can be explained only by a corresponding amount of mass, or matter. Some undiscovered mass — dark matter — must exist in order to explain gravity, but no one has seen any traces of it. Those traces, when they are finally found, will be exotic particles left over from the Big Bang. In the tale we tell about everything we know, scientists have now brought us to the edge of the deep, dark woods. They, and we, are waiting eagerly to see how the rest of the story goes.”

Ok, out comes the professorial red pen.

First, a relatively minor point of order. “…the force of gravity in the universe can be explained only by a corresponding amount of mass, or matter…” This isn’t great writing, because mass and matter are not the same thing. Matter is a type of substance. Mass is a property that substance (including ordinary matter, such as tables and planets) can have. Mass and matter are as different as apples and applets. You can read about these distinctions here, if you like. The author is trying to evade this distinction to keep things simple: the more correct statement is that gravity (in simple circumstances) is a force exerted by things (including ordinary matter) that have mass.

But here’s the real offending remark: “Some undiscovered mass — dark matter — must exist in order to explain gravity, but no one has seen any traces of it.” Dark matter is most certainly not needed to “explain gravity” in some general way; there’s not one bit of truth in that remark. For instance, the gravitational pull of the sun on the earth (and vice versa), and the pull of the earth on you and me (and vice versa), has absolutely nothing whatsoever to do with dark matter, nor is dark matter needed to explain it.

What the author should have said is: since the 1960s we have known that gravitational forces on large astronomical scales seem to be stronger than we can account for, and so either our equations for gravity are wrong or there is matter out there, pulling on things gravitationally, that we cannot see with any type of telescope. The reason the latter possibility is taken more seriously than the former by most experts is that attempts to modify gravity have not led to a convincing case, while the evidence for additional “dark” matter has grown very strong over recent decades.

Here’s one of the several arguments that suggest the possibility of dark matter… the simplest to explain. Experts study the motions of the stars in our own galaxy — the star city known as the Milky Way — and also study the motions of stars in other galaxies. [The overall motions of galaxies themselves, inside giant clusters of galaxies which can be found in deep space, are also studied.] Now what we ask is this; see Figure 1. Supposing all of the matter that is out there in the universe is of a type that we can see in one way or another: stars, gas, dust of various types. Then we can figure out, just by looking with a telescope and doing simple calculations, roughly how much measurable matter is in each galaxy, how much mass that matter has, and where it is distributed inside the galaxy. We can next use that information to figure out how hard that matter pulls on other matter, via the force of gravity. And finally — crucially! — we can calculate how fast that pull will make the matter move, on average. And what do we find when we measure how fast the stars are moving? Our calculations based on the matter that we can see are wrong. We find that the stars in the outer edges of a galaxy, and the galaxies inside clusters, are moving much, much faster than our calculation predicts. (This was discovered in the 1960s by Vera Rubin and Kent Ford.) It’s as though they’re being pulled on by something unseen — as though the gravity on the stars due to the rest of the galaxy is stronger than we’ve guessed. Why is this happening?

Fig. 1: One of several lines of evidence in favor of the hypothesis of dark matter is that stars in the outer regions of galaxies move much faster than would be the case if the galaxy was made only from what we can see.

One possibility is that there is matter out there that we can’t see, a lot of it, and that matter is inside galaxies and inside clusters of galaxies, exerting a pull that we haven’t accounted for properly. A huge “halo” of dark matter, in this view, surrounds every galaxy (Figure 2).

Clearly, this isn’t the only logical possibility. Another option is that there could be something wrong with our understanding of gravity. Or there could be some other new force that we don’t know about yet that has nothing to do with gravity. Or maybe there’s something wrong with the very laws of motion that we use. But all attempts to make sensible suggestions along these lines have gradually run into conflicts with astronomical observations over the recent decades.

Fig. 2: The visible part of every galaxy is believed to lie roughly at the center of a much larger halo of dark matter.

Meanwhile, during those last few decades, a simple version of the “dark matter” hypothesis has passed test after test, some of these tests being very complex and subtle. For example, in Einstein’s theory of gravity, gravity pulls on light, and can bend it much the same way that the lenses in eyeglasses bend light. A galaxy or galaxy cluster can serve to magnify objects behind it, and by studying these lensing effects, we again conclude there’s far more matter in galaxies and in clusters than we can see. And there are other arguments too, which I won’t cover now.

So while an explanation for the fast motion of stars inside galaxies, and galaxies inside clusters, isn’t 100% sure to be dark matter, it’s now, after many years of study, in the high 90%s. Don’t let anyone tell you that scientists rushed to judgment about this; it has been studied for decades, and I can tell you from experience that there’s a lot more consensus now than there was when I was an beginning undergraduate 30 years ago.

“Those traces, when they are finally found, will be exotic particles left over from the Big Bang.” Will they? Will the dark matter turn out to be particles from the Big Bang? Not necessarily. We know that’s one possibility, but it’s not the only one. Since I explained this point last week, I’ll just refer you to that post.

Now here come the big meta-questions: should the New York Times be more careful about what it puts on its editorial page? Should its editors, who are not scientists, talk broadly about a subtle scientific topic without fact-checking with an expert? What are the costs and benefits when they put out oversimplified, and in some ways actually false, information about science on their editorial page?

Personal and professional activities require me to take a short break from posting. But I hope, whether you’re a novice with no knowledge of physics, or you’re a current, former, or soon-to-be scientist or engineer, or you’re somewhere between, that you can find plenty of articles of interest to you on this site. A couple of reminders and pointers:

* If you haven’t yet seen my one-hour talk for a general audience, “The Quest for the Higgs Boson”, intended to explain accurately what the Higgs field and particle are all about, while avoiding the most common misleading short-cuts, it’s available now, along with a 20-minute question and answer session.

* If you want a slightly more technical and written discussion of the Higgs field and particle, complete with animated images, and suitable for people who may once have had a semester or two of university physics and math, try this series of articles first, and then go to this series.

* If you’d like to better understand the language of “matter”, “mass”, and “energy” that is everywhere in popular explanations of science, but eternally confusing because of how different authors choose to talk about these subjects, you might find some useful tips in these articles: #1, #2, #3, #4.

* If you need a reminder about what “ordinary matter” (i.e. things like pickles, people and planets) is made of, try this series, which goes all the way from molecules down to quarks.

Hopefully there’s something on that list that interests you, and many links within those articles to other things that may even interest you more. Have fun exploring! And stay tuned; I’ll be writing more in the near future…

A deeply unfortunate case, the subject of today’s post, is the word “mass”. Mass was confusing before Einstein, and then Einstein came along and (accidentally) left the word mass with two different definitions… both of which you’ll see in first-year university textbooks. (Indeed, this confusion even extended to physicists more broadly, causing the famous particle physicist Lev Okun to make this issue into a cause celebre…) And it all has to do with how you interpret E = mc² — the only equation everybody knows — which relates the energy stored in an object to the mass of the object times the square of the universal speed limit c, also known as “the speed of light”.

Posts have been notably absent, due mainly to travel with very limited internet; apologies for the related lack of replies to comments, which I hope to correct later this week.

Meanwhile I’ve been working on a couple of articles related to the nuclei of atoms, part of my Structure of Matter series, which serves to introduce non-experts to the basics of particle physics. The first of these articles is done. In it I describe why it was so easy (relatively speaking) to figure out that nuclei are made from certain numbers of protons and neutrons, and how it was understood that nuclei are very small compared to atoms. Comments welcome as always!

A related article, which should appear later this week, will clarify why nuclei are so tiny relative to atoms, and describe the force of nature that keeps them intact.

My rather hasty, breathless and inconsistent summaries (#1, #2 and #3) of last week’s talks at the excellent Higgs Symposium (held at the University of Edinburgh, as part of the new Higgs Center for Theoretical Physics) clearly had their limitations. So I thought it might be useful to give a more organized overview, with more careful language appropriate for non-expert readers, of our current knowledge and ignorance concerning the recently discovered Higgs-like particle (which most of us do believe is a Higgs particle of some type, though not necessarily of the simplest, “Standard Model” type.)

This is a post about constancy and inconstancy, one of my favorite topics. And about how alcohol can make you smarter.

There are many quantities that we call “constants of nature”. Of course, anything we call a “constant” is merely something that, empirically, appears to be constant, to the extent we can measure it. Everything we know comes from observation and experiment, and our knowledge is always limited by how good our measurements are.

We have pretty good evidence that a number of basic physical quantities are pretty much constant. A lot of evidence comes from the constancy of the colors of light waves (i.e. the frequencies of waves of electromagnetic radiation) that are emitted by different types of atoms, which appear to be very much the same from day to day and year to year and even across billions of years (neat trick! will describe that another time), and from here to the next country and on to the moon and to the sun and across our galaxy to distant galaxies. For example, if the electron mass changed very much over time and place, or if the strength of the electromagnetic force varied, then atoms, and the precise colors they emit, would also change. Since we haven’t ever detected such an effect, it makes sense to think of the electron mass and the electromagnetic force’s strength as constants of nature.

But they’re not necessarily exactly constant. One can always imagine they vary slowly enough across time or place that we wouldn’t have noticed it yet, with our current experimental technology. So it makes sense to look at very distant places and measure whatever we can to seek signs that maybe, just maybe, some of the constants actually vary after all.

Suppose they did vary? Well, the discovery of any variation whatsoever, in any quantity, would be a bombshell, and it would open up a door to an entirely new area of scientific research. Once one quantity were known to vary, it would be much more plausible that others vary too. For instance, if the electron mass varies, why not the W particle’s mass, which affects the strength of the weak nuclear force, and thereby radioactivity rates and the properties of supernovas? If the electromagnetic force strength varies, why not that of the strong nuclear force? There would be interest in understanding whether the variation is over space, over time, or both. Is it continuous and slow, or does it occur in jumps? One can imagine dozens of new experiments that would be proposed to study these questions — and the answers might reveal relations among the laws and “constants” of nature that we are currently completely unaware of, as well as giving us new insights into the history of the universe.

So it would be a very big deal. [Though I should note it would also be puzzling: even small variations in these constants would naively lead to large variations in the “dark energy” (i.e. cosmological “constant”) of the universe, which would potentially make the universe very inhomogeneous. However, we don’t understand dark energy, so this expectation might be too naive.] Since there’s no story about it on the front page of the New York Times, you can already guess that no variation’s been found. But a nice new measurement’s been done. Continue reading →

Who’s at fault here, and how did this happen? There’s plenty of blame to go around; some lies with the journalist, who would have been wise to run his prose past a science journalist buddy; some lies with the editors, who didn’t do basic fact checking, even of the non-science issues; some lies with a public that (broadly) doesn’t generally care enough about science for editors to make it a priority to have accurate reporting on the subject. But there’s a history here. How did it happen that we ended up a technological society, relying heavily on the discoveries of modern physics and other sciences over the last century, and yet we have a public that is at once confused by, suspicious of, bored by, and unfamiliar with science? I think a lot of the blame also lies with scientists, who collectively over generations have failed to communicate both what we do and why it’s important — and why it’s important for journalists not to misrepresent it. Continue reading →

Robert Garisto sent me a reply to my previous post; here it is. [A “vev” is shorthand for a non-zero value in the vacuum of space, what I call a “non-zero average value”.]

—

Matt – Thanks for your extensive reply to my comment! Of course I agree that a scalar field without a vev can have a hard mass term. And I do agree that how the Higgs boson gets mass is at least somewhat different than how the W does.

Let’s agree to define a Higgs as a scalar field with a vev. Then I think you agree that the mass of the excitation about the vev, the Higgs boson, is not a hard mass term, one obtains it by finding the minimum of the potential as you did above. Now if there are other scalars with vevs, the mass of the Higgs boson we are concentrating on can depend on those too. But isn’t it correct to say that the mass of such a Higgs boson goes to zero in the limit that all of those vevs go to zero? If so, I would say that the Higgs boson mass is provided by the Higgs fields (all scalars with nonzero vevs).

Anyway, the main reason I made the comment is that for the purposes of explaining to the public electroweak symmetry breaking, I think it makes sense to say that the Higgs boson mass comes from the Higgs field, because it is, in the SM, proportional to the vev. It’s also kind of neat, I think.

—

We disagree, that’s all there is to it. What Garisto says about the Standard Model is a simple consequence of dimensional analysis, not a fundamental relation that applies widely. And no, it is not correct to say that the mass of a Higgs boson always goes to zero in the limit that all vevs go to zero; there can be first order phase transitions in which, as the parameters change, the Higgs field’s vev jumps from non-zero to zero abruptly, and the mass of the Higgs particle is never zero. So I think to tell the public that the Higgs particle gets its mass from the Higgs field is to confuse them into thinking that the Higgs particle gets its mass the same way the other known particles do — which is false.

But in any case, we agree it’s not that big a deal. The thing which is important for the public to understand is that the Higgs field does not give mass to all massive objects — such as atomic nuclei and black holes. And the thing which it is important for particle physics students to understand is that the Higgs mass is not generically proportional to the vev of the Higgs field.

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A Higgs particle is produced in a proton-proton collision at center, and decays to two photons (particles of light, indicated by green towers) in an LHC detector. Tracks emerging from center are from remnants of the two protons.